Welcome to HVAC-Talk.com, a non-DIY site and the ultimate Source for HVAC Information & Knowledge Sharing for the industry professional! Here you can join over 150,000 HVAC Professionals & enthusiasts from around the world discussing all things related to HVAC/R. You are currently viewing as a NON-REGISTERED guest which gives you limited access to view discussions

To gain full access to our forums you must register; for a free account. As a registered Guest you will be able to:

Participate in over 40 different forums and search/browse from nearly 3 million posts.

Eliminating a restriction on an empty system.

The situation is that I have an empty split system. That is, it has
no refrigerant in it. It is a TXV system. I want to be sure it has
no restrictions when I re-charge and run it. I don't have a lot
of extra time, so I need to do this as efficiently as possible.
(Yes, I am covering for a diagnostic screw-up when the system
was charged. I have another post around here detailing that
mistake.)

I have never encountered a restriction so anything anybody can
tell me about actually encountering them would be welcome.

I have heard mention of liquid line restrictions, but how
big of a restriction would it have to be to clog a refrigerant line?
Aren't we left with filter-drier, TXV, and maybe (?) evaporator
coils lines? (Can they be tiny or is that just for cap-tube systems?)
Do typical split system residential evap coils have
tiny lines and multiple circuits or are they generally one or two
circuits with relatively non-cloggable lines?

I was thinking that I could front seat both service valves
and run nitrogen through my gauges into the liquid line
and let it open at the suction line? If there were a restriction
affecting suction pressure, would I know it doing this
nitrogen flush? If a filter drier were clogged, would this
nitrogen flush do more harm than good?
Should I open up the line at the TXV
for inspection? Should I go into nursing?
Again, anything would be more than I know now.

Is this the same unit with the questionable Lps? Is it open now with no pressure in the system, does it close now when you pressurize the system?

Just throwing my 2cents out there!

Your 2 cents are valuble, and you are correct, sir This is the
same one with the questionable LPS. I didn't have much time to
play with the pressure switch, but I believe it did close at around
50 psi as I was putting nitrogen in it a second time. (I had
pressurized it to about 225 psi for the leak check, but then tried
to flush the refrigerant/nitrogen combo for a standing-pressure
test, but ran out of nitrogen, so left it with about 70 psi of nitrogen
in it. Yes, I wish I carried a spare bottle of nitrogen like I get to
carry spare bottles of oxygen and acetylene. I had thought my
nitrogen was almost full. I suspect somebody borrowed it in my
absence and neglected to leave a note or let me know. Anyway.....)

Another tech had been out there last summer and encountered
a low pressure switch, but played with it---details are sketchy--
and noted on the invoice that everything seemed okay. Maybe
the low-pressure switch is flakey? Can restrictions be intermittent?
As I said, this is uncharted territory for me. My textbook never
really said much useful about restrictions, other than detection
via temperature/pressure drops. Of course, those would be with
a charged & operating system. At this point, I am somewhat
committed to soldering in the new pressure switch, evacuating,
& charging. So anything else I do needs to be done quick!
As I said, I was lead (?) to assume a leak after reading two
old invoices before even visiting this system. After recovering
the charge and noting that it weighed the nameplate amount,
I was left with suspecting the low-pressure switch. This pressure
to commit to a diagnosis when I'm not sure is what I like least
about HVAC. I like it MUCH better when my diagnostic
procedures leave me little or no doubt that my diagnosis is
correct. Unfortunately, restrictions are so far uncharted for me.
This is one of those big learning periods!
I am also wondering if the the wire connector from the pressure
switch at the board is maybe suspect. These wires are
not like simpler systems where all of the wires run into one
sturdy, large connector. This is a fancier board with only two wires
on the flimsy connector to the LPS..

It always amazes me how much clearer my thinking is the
night (and days) AFTER a service call. Yes, part of my
problem here is the amount of time my boss
and the customer already have into this call. I wish I could
start it over but that is not my reality. Man, next time......

Anyway, for anbody reading and not familiar with the other
post/thread of mine, the unit went off on the LPS but the
charge was correct. As somebody mentioned, there are
other reasons for low suction pressure rather than a low charge.
Air-handler was on. Filter was clean. Static pressure seemed
reasonable. No good reason for the unit to go off on the LPS
within about three minutes of switching on the A/C. So I am
left suspecting the LPS or a restriction, hence my anxiousness
to detect and maybe clear any restriction as I solder, pressure-test,
evacuate, and charge. I really don't have time to charge and
run the system for further diagnosis---unless, as they say,
I have time to do it over, not having had the time to do it right the
first time! I hope this turns out well.

Not enough time ?????
every call is your only call till you are done IMHO this is what i believe and feel for 30 something years
forget you other calls and the pressure!! your costumer is your only priority

fill the system and diagnos it. Will take you far less time then trying to save time guessing. Forget all other invoives on the system and and start with a fresh mind and make your own determination of what is hapening.

nitrogen will not tell you much except if you have a leak

you need a running system to see what is realy happening and fix the problem from there

You type a lot and no where is a model number, or even a brand of this system with the 'fancy board'.
I've seen these boards go bad, did you remove it from the mounts and look closely at the circuits? The ones I see usually have a corroded board, seen them like this just over a year old too.

You say you have an open system, you should replace the drier anyway or remove it. Take the TXV screen in the liquid line out and make sure it's clean, with the screen out is a good time to flush out the liquid line.

Put it all back together, pressure test, evacuate, weigh in the charge and adjust it for line set length. It should work fine.
Then to check for a restriction, access the cap tubes going to the txv distributor and check the temperature of each one while it's running. There should not be more than an 6-8 degree difference.

Don't forget to check the blower motor, could have a bad cap causing it to overheat and shut off when you're not there. It could just be going bad or it could be something that controls the blower motor.

I found a problem a few days ago, the motor was just shutting off for no apparent reason and the evap would start to ice up. I found a splice in the low votage wire between the a/h and t-stat, the wire nut on the green wire was loose. That call was for a clogged drain, while I was looking at a clean drain I could hear the motor cutting off for a split second, eventually it shut of and stayed off. I'm confident there was nothing wrong with the drain and it had just iced up, over heated the compressor, and when I got there it had melted and was mostly dryed out.

If this system had a low pressure switch it would have shut down. If it had a board it would have stored a low pressure fault.

The method of purging nitro through would work in the case of a total block. But many different factors could indicate a restriction or a faulty metering device (e.g. txv). Moisture can also cause this problem. It's always good to have a sight glass for this reason. The only definite way is to charge the system and check your pressures. If you want to eliminate a clog in the evap force the evap to freeze. All tubes should freeze at the same time and evenly.

Cycling on low pressure switch?, there is a screen on thr inlet of the txv, this can clog with trash causing your problem. If txv can be removed, clean it, if it's brazed in, you are going to need (a least I would) to replace the txv. Sounds like your boss is a shade tree, some "snake bit" jobs happen. This sounds like an old problem, never fixed problem.

I once found a 3/8 bolt in the suction header(from factory) of a 3 ton evap coil. Think about that one girls.

Forgot to add, if you can pump down the sys, weigh in charge, run cond with the evap fan OFF. Frost coil and see where the ice starts on the txv, should be near middle of valve. Bypass lps for this.
E

Makes me think this is a new or somewhat new install under warranty, thats why the boss is rushing him. Our work comes with a one year labor warranty.

Worst thing I ever found was the contents of the filter drier in the entire liquid line. It didn't have a very long run time before the lps shut it down. All I did was cut the drier out, disconnect the txv, remove the screen, blow the line out.

And by screen in front of the txv I'm refering to the small one that goes into the 3/8 line, not the kind in the bulb. If theres a problem with the type of screen in the bulb it needs to be cut out. What you replace it with is your choice, a new one or a piece of tubing.

Not enough time ?????
every call is your only call till you are done IMHO this is what i believe and feel for 30 something years
forget you other calls and the pressure!! your costumer is your only priority

fill the system and diagnos it. Will take you far less time then trying to save time guessing. Forget all other invoives on the system and and start with a fresh mind and make your own determination of what is hapening.

nitrogen will not tell you much except if you have a leak

you need a running system to see what is realy happening and fix the problem from there

make the time or walk away

Had a customer call me for a second opinion behind a guy I used to have a good deal of respect for. After I fixed the furnace(took me almost two hours to find what the problem was) I ran into the first contractor one day in town. He had been called by the customer to let them know I had fixed the problem they told them could not be fixed and would just need to be replaced. He asked me what I had found and I told him it was electrical and that it only did it once it a very great while but would lock the board out at times. He said he did not have the time to find problems like that. And it floored me. !!!! I asked him, "aren't you a service company"? Is the customer not paying you to find and fix the problem? I just could not believe he said that to me. I guess that is the reason he sells so much more equipment than me.

Bad information is worse than no information at all.

There are three kinds of people in this world. Those who can count and those who can't!

I've been out all day, so just got back to read all of the responses.
Wow. Lots of valuable information, none of it mentioned in my
textbook or in school.

This job started out as a PM, and we have the model/serial info
on the sheets given to us, otherwise I would have put them in my notebook
that I always bring home. It was a Bryant. I remember nothing
about the model number. But the main problem here was that I didn't
take the time to properly troubleshoot the low pressure, so I had
no useful information to make the model number very useful.
Really, the general information about troubleshooting restrictions
and where they most likely occur is plenty valuable.

The time thing is a bit complicated in this case. It is probably best
if I keep mum on it, but I am taking to heart all of your comments
and I am aiming to have it never happen again.

Again, I sincerely appreciate you guys taking time out to
clue me in. It does make me wonder how new techs learned
things before the web or this forum.